The present invention relates to combining and separating a low data rate digital channel with or from the high data rate digital channel of a transmission link by overmodulation or amplitude superposition applied to a signal transmitted on the line.
In this type of operation which is commonly performed for adding a service path to a digital transmission link, the overmodulation or amplitude superposition factor must be chosen in such a manner as to obtain the same error rate performance in the high data rate digital channel and in the low data rate channel, while nevertheless ensuring that said factor remains sufficiently low for the major portion of the power transmitted to line to remain dedicated to the high data rate digital channel.
In most conventional transmission links, the signal of the high data rate digital channel does not interfere with the signal of the low data rate digital channel since it is encoded using a redundant transmission code such as the 5B6B or the 12B1P1C codes, etc., which facilitate clock recovery and error detection on the line and which also have the property of eliminating the D.C. component and of attenuating low-frequency components. This means that a low overmodulation or superposition factor may be used and the low data rate channel can be separated merely by lowpass filtering.
In recent very high data rate transmission links such as optical fiber transmission links, it has become necessary to abandon transmission encoding for the high data rate digital channel since it suffers from the drawback of further increasing the data rate. In the absence of such encoding, the high data rate digital channel has low-frequency components and these constitute major interference for the low data rate digital channel, and they are countered by increasing the overmodulation or amplitude superposition factor, and consequently by increasing the fraction of the transmitted power which is dedicated to the low data rate digital channel.